Medications to treat and prevent cardiovascular disease (CVD) have substantially reduced CVD-related morbidity and mortality; however, their diffusion has been uneven. Physicians have been slow to adopt some evidence-based treatments and have rapidly adopted others of low value. Use of newer, brand name drugs is much higher in some regions in the US and suggests that physicians in those regions are rapid adopters of new drugs. Yet the determinants of physician adoption of new medications are poorly understood. Physicians are likely to turn to their peers for information on drugs' uses, benefits and risks. Network analysis offers an analytic framework and measurement techniques for understanding how physicians learn from each other about new treatments. Our long-term objective is to understand the role of physician networks in technology diffusion. To accomplish this objective, we propose two specific aims. First, we will examine the association at the local-level between physician adoption of new medications and prescription drug spending and total non-drug medical spending. We will measure the rate and speed of adoption among nearly all physicians in Pennsylvania of 5 new drugs used for CVD or diabetes introduced in recent years. We will obtain data on prescribing for nearly all of Pennsylvania's 50,000 physicians. We will also obtain Medicare and Medicaid claims data for the more than 2 million adults in the state who are covered by these programs. Second, we will examine the influence of multiple types of professional networks, physician characteristics, patient characteristics, and institutions on physician adoption of new medications. In particular, we will assess the influence on physician adoption of new drugs of physician networks formed during training (e.g., residency program), in organizations (e.g., medical groups, hospitals), and through the sharing (referral) of patients. Understanding the role of these networks will guide dissemination of interventions such as academic detailing to improve the quality and efficiency of pharmacotherapy. Medicare and Medicaid are key settings for studying network effects because they provide coverage to the most vulnerable and because the costs of these programs exert substantial pressure on public budgets.